The present invention generally relates to a system for reducing noise in an electrical signal and more particularly relates to a method and a circuit arrangement for processing signals for an active motion sensor which generates a sequence of input pulses.
Active motion sensors are generally comprised of a pick-up for measuring data with a magnetostatically sensitive element with a permanent magnet (sensor element) which is coupled magnetically to an encoder (measuring data emitter) that executes a motion being sensed. The sensor element responds to a change in the flux density or the field vector which is induced by this motion. The output signal of the sensor element is sent to a modulator which either generates a periodic sensor voltage or a periodic sensor current, and the frequency is determined by the motional speed.
A special advantage of these active sensors compared to prior art passive sensors includes that the voltage of the sensor signal developed is irrespective of the motional behavior and, especially, the motional speed so that level adaptions and protection circuits are virtually unnecessary. Active motion sensors are increasingly employed for this reason. Depending on the design, a pulse voltage or a pulse current is generally produced which is sent as a sensor signal to a signal processing device to be conditioned and evaluated and, thereafter, transmitted as a motion signal to a traction or brake slip control system.
However, provisions must be made in this arrangement to prevent noises and, in the event of noises occurring nevertheless, prevent reactions which might jeopardize the vehicle""s operation. In this respect, a distinction must be made between noises which cause a total failure of the sensor signal and noises which influence this signal in such a manner that the affected control no longer reacts in a way appropriate for the actual vehicle operating condition. These last-mentioned noises may be caused, for example, by ignition pulses, ESD discharges, and other signals, the duration of which may amount up to a third of the duration of the useful signal. The result of the noises is that by superposition on the sensor signal the said""s frequency is temporarily changed, and a change in the motional speed is thereby indicated erroneously by the signal processing device.
Because a distinction between a change in the sensor signal due to an actual change of the motional behavior and a noise-induced change which impairs the sensor signal is difficult and intricate to make, one has so far obviously refrained from taking precautions for preventing a possible misinterpretation of the sensor signal.
An object of the present invention is to provide a method and a circuit arrangement for processing signals for an active motion sensor of the type referred to hereinabove which permits preventing the effects of noises that occur only temporarily and in a way that might lead to misinterpretation of the sensor signal.
This object is achieved by a method for processing signals for an active motion sensor of the type referred to hereinabove which is characterized in that each input pulse of a pulse train is integrated and an associated output pulse is generated during a period in which the integrated signal is in excess of a predeterminable second threshold value after a predeterminable first threshold value is exceeded so that the output pulse has a time delay with respect to the input pulse.
Further, this object is achieved in a circuit arrangement for processing signals for an active motion sensor of the type referred to hereinabove which includes at least one integrating filter circuit by which each input pulse of a pulse train is integrated and an associated output pulse is generated during a period in which the integrated signal is in excess of a predeterminable second threshold value after a predeterminable first threshold value has been exceeded so that the output pulse has a time delay with respect to the input pulse.
The advantage of these solutions is that noises which are shorter than the delay time will extend this time only slightly and can be rated as minor speed variation within a tolerable range. Only those noises which last longer than the delay that is due to the integration can cause a fault which, however, will only occur with this time delay.
Accordingly, the integrating filter circuit may be an analog filter with a capacitor and at least one current source to which can be sent the input pulse for charging the capacitor and for generating the integrated signal, and the first and the second threshold value is in each case defined by a predetermined voltage value which declines at the capacitor.
As an alternative, the integrating filter circuit may also be a digital filter with at least one counter, to which the input pulse can be sent for activation, wherein the integrated signal represents a counter reading, and the first and the second threshold value is respectively defined by a predetermined counter reading that is to be reached.